Dilated cardiomyopathy is one of the myocardial diseases characterized by progressive depression of myocardial contractile function and dilated ventricle, and the dysfunction of depressing dilated left ventricle is present. This disease often develops chronically and the prognosis is poor in the majority of cases. Therefore, in Europe and the United States, cardiac transplantation is necessary for this disease in many cases, and above 90% of cardiac transplantation cases in Japan were those of patients with this disease. 30% of dilated cardiomyopathy patients are said to have a congenital mutation in the gene that codes an important component of myocardium, which links cytoskeleton to cell matrix. However, the causes of the rest of cases are unknown. In both cases, the onset rate is very high (36.5 in 100,000 people) and the death rate is also high. However, there is no effective therapy except for cardiac transplantation.
In addition to ischemic, toxic, metabolic, infectious, and genetic causes, autoimmunity has been suspected to be one of the main causes. Several papers have reported that the patients of myocarditis or cryptogenic dilated cardiomyopathy have heart reactive autoantibodies. Antibodies to the mitochondrial ADP/ATP translocator or β1-adrenoceptor, which are often found in human patients, have been shown to enhance Ca2+ current (Journal of Experimental Medicine, 1988, 168(6), p 2105-2019, European Journal of Pharmacology, 2001, 423(2-3), p 115-119). In addition, immunoabsorption has been reported to have beneficial effects. (Circulation, 2001, 103(22), p 2681-2686). Stephan B. Felix proved that the therapy with immunoabsorption made patients' hemodynamics improved and the column eluent, which contains harvested antibodies from immunoabsorption column, decreased cell contraction in myocardial cells because of calcium current suppression (Journal of the American College of Cardiology, 2002, 39(4), p 646-652). Experimental studies in rodents have shown that cardiotropic viral infections or immunization with cardiac antigens can elicit injury of cardiomyocytes, leading to cardiomyopathy with concomitant production of heart-reactive autoantibodies and cytotoxic T cells (Circulation, 1982, 65(6), p 1230-1235, Clinical Immunology and Immunopathology, 1987, 43, 1, p 129-139, Journal of Molecular and Cellular Cardiology, 1997, 29(2), p 641-655, Journal of Immunology, 1987, 139(11), p 3630-3636). However, because autoimmune responses observed in the rodent models and human patients could be secondary to heart inflammation, the involvement of autoimmunity in the pathogenesis of dilated cardiomyopathy is still debatable (Journal of Immunology, 1990, 145(12), p 4094-4100).
PD-1 is an immune inhibitory receptor belonging to CD28/CTLA-4 family and inhibits antigen receptor-mediated signaling by recruiting SHP-2 upon engagement with its ligands, PD-L1 or PD-L2 (Current Opinion Immunology, 2002, 14(6), p 779-782). C57BL/B6 PD-1-deficient mice developed lupus-like glomerulonephritis and arthritis (Current Opinion Immunology, 2002, 14(6), p 779-782). The inventors have recently shown that BALB/c PD-1-deficient mice develop dilated cardiomyopathy (Science, 2001, 291(5502), p 319-322, WO02/39813).
The dilated heart of the mice showed prominent depositions of immune complex on myocardial cells. Furthermore, sera from PD-1-deficient mice contained high-titer autoantibodies against a heart-specific, 30-kDa protein. Although these results support the hypothesis that autoimmunity is a probable cause of dilated cardiomyopathy, there was no direct evidence. Therefore, it is essential to identify the 30-kDa antigen and assess the pathogenic role of the antigen-specific autoimmune reaction in dilated cardiomyopathy.